In precision press operations, more particularly when articles compacted of powder material are subject to tight tolerances and high precision, in the dimensions as well as in the density of the compacted articles, it is advantageous to provide some mechanism for maintaining a constant press force for each stroke of the press, and for cushioning the impact shocks. Various types of deflection compensation systems and impact shock cushioning devices have been proposed in the past, such as disclosed in U.S. Letters Patent No. 3,733,154, assigned to the same assignee as the present application, which discloses an impact shock or absorbing system forming part of the press ram itself. Deflection compensation and shock absorbing mechanisms may also be incorporated into the die and punch assembly, as disclosed, for example, in U.S. Letters Patent No. 3,669,582, also assigned to the same assignee as the present application.
In addition, it is often desirable to effectuate a pressing operation at a predetermined ram pressure or at ram pressure within a predetermined range. Such a requirement is difficult to achieve in a mechanical powder compacting press because of the variations that may occur in the volume of powder material used in consecutively filling the die cavity. There is a requirement in some powder material compacting operations for obtaining articles of a predetermined constant density from article to article, while the height or thickness of the article may be allowed to vary from article to article within a relatively wide range. Examples of such articles wherein constant density is the predominant characteristic, rather than dimensions, are nuclear fuel pellets used to charge reactor rods. The rods are tubular and relatively slender, and they are of substantial length. The principal requirements are that all the pellets be of the same diameter and the same density. The diameter of the pellets correspond to the internal diameter of the rods, and any variation in thickness of the pellets is compensated for, in view of the length of the rods, by loading each rod with the number of pellets required to fill the rod.